1. Field of the Invention
The present invention relates to a system and method for degassing air dissolved in a liquid.
2. Description of the Related Art
In the formation of a photosensitive planographic printing plate, a coiled aluminum plate (hereinafter referred to as “web”) normally undergoes surface treatments such as graining, anode oxidation, or chemical conversion treatment either independently or in combination, next undergoes a coating process in which a coating liquid is applied, and then progress to a drying process.
In the process in which coating liquid is applied, if the coating liquid containing dissolved air is applied in that state, air bubbles cause the coated surface to have defects such as bubble repellent marks, vertical streaks, or pinholes, and it is not possible to form a uniform photosensitive layer on the web. Therefore, it is necessary to remove the air dissolved in the coating liquid which is to be applied on the web.
As is shown in FIG. 7, a normal method for effectively removing air dissolved in a solution is for a coating liquid L stored in a reserve tank 100 to be fed by a liquid feeding pump 102 to degassing devices 104 which are arranged in rows, for the air dissolved in the liquid to be removed and for the liquid to be further fed in that state to a coating device 108 through a filter 106 (See Japanese Patent Application Laid-open (JP-A) No. 9-253459).
However, the amount of dissolved air in the coating liquid which is stored in the reserve tank 100 fluctuates, depending on changes in the amount of the coating liquid returned to the reserve tank 100 from the coating device 108, on the amount of coating liquid fed from the degassing devices 104 to the coating device 108, on the timings that coating liquid is fed from the stock tank 120 to the reserve tank 100, and on other conditions. For these reasons the amount of dissolved air in a coating liquid that has been degassed also fluctuates, whereby coating failures may be generated in the coating device. To cope with these changes in load, plural degassing devices 104 are arranged so as to minimize fluctuations in the amount of dissolved air.
However, when plural degassing devices are arranged and particularly when film degassing devices are used, the performance of the devices deteriorates as the state of degassing films 105 deteriorates, and it is necessary to replace the degassing films 105 periodically, thereby increasing running costs.
Since different methods of coating are used for different types of products, requirements in respect of the amount of dissolved air also vary from one coating liquid to another. However, conventional degassing devices only degas at constant degree of vacuum by driving a vacuum pump 110. In other words, they are restricted to a fixed set of conditions, and it has not proved possible to regulate flexibly the amount of dissolved air in a coating liquid that has been degassed.